Sprinkler systems are used in buildings as fire protection measures. There are three basic types of fire protection sprinkler systems: wet pipe systems, dry pipe systems and deluge systems.
Wet pipe systems are more common than all other types of sprinkler systems. They are the most reliable because they are simple, with the only operating components being automatic sprinkler heads and (commonly, but not always) an automatic alarm check valve. An automatic water supply provides water under pressure to the system piping.
Dry pipe systems are generally installed in spaces in which the ambient temperature may be cold enough to freeze the water in a wet pipe system, rendering the system inoperable. Dry pipe systems are most often used in unheated buildings, in parking garages, in outside canopies attached to heated buildings (in which a wet pipe system would be provided), or in refrigerated coolers. Dry pipe systems are the second most common sprinkler system type. In a dry pipe system, water is not present in the piping until the system operates. The piping is filled with air below the water supply pressure. To prevent the larger water supply pressure from forcing water into the piping, a dry pipe valve (a specialized type of check valve) provides a greater force on top of the check valve clapper by use of a larger valve clapper area exposed to the piping air pressure, as compared to the higher water pressure but smaller clapper surface area. When one or more of the automatic sprinkler heads is exposed, for a sufficient time, to a temperature at or above the temperature rating, it opens, allowing the air in the piping to vent through that sprinkler head. Each sprinkler head operates individually. As the air pressure in the piping drops, the pressure differential across the dry pipe valve changes, allowing water to enter the piping system. Water flow from sprinkler heads is delayed until the air is vented from the sprinkler system piping. Dry pipe sprinkler systems may be advantageous for protection of valuable collections and other water sensitive areas. In a wet system, piping may slowly leak water without attracting notice, while dry pipe systems might not fail in this manner. However, dry pipe systems require additional control equipment and air pressure supply components which increases system complexity. This puts a premium on proper maintenance, as this increase in system complexity results in an inherently less reliable overall system (i.e., more single failure points) as compared to a wet pipe system. The added complexity also impacts the overall dry pipe installation cost, and increases maintenance expenditure primarily due to added service labor costs. Further, regulatory requirements limit the maximum permitted size of individual dry pipe systems, unless additional components and design efforts are provided to limit the time from sprinkler system activation to water discharge to under one minute. These limitations may increase the number of individual sprinkler zones (i.e., served from a single riser) that must be provided in the building, and impact the ability to make system additions. Furthermore, because the piping is empty at the time the sprinkler system operates, there is an inherent time delay in delivering water to the sprinkler heads which have operated while the water travels from the riser to the sprinkler, partially filling the piping in the process. This delay in fire suppression results in a larger fire prior to control, increasing property damage. Following operation or testing, dry pipe sprinkler system piping is drained, but residual water collects in piping low spots, and moisture is also retained in the atmosphere within the piping. This moisture, coupled with the oxygen available in the compressed air in the piping, increases pipe internal wall corrosion rates, possibly eventually leading to leaks. The internal pipe wall corrosion rate in wet pipe systems (in which the piping is constantly full of water) is much lower, as the amount of oxygen available for the corrosion process is lower. Corrosion can be combated with galvanized steel pipe which is less susceptible to corrosion, or by using dry nitrogen to pressurize the system rather than air. These additional precautions increase the cost of the system, but can help prevent system failure and premature need for system replacement.
Deluge systems are systems in which all sprinkler heads connected to the water piping system are open. These systems are used for special hazards where rapid fire spread is a concern, as they provide a simultaneous application of water over the entire hazard. They are sometimes installed in personnel egress paths or building openings to slow travel of fire. Water is not present in the piping until the system operates. Because the sprinkler head orifices are open, the piping is at atmospheric pressure. To prevent the water supply pressure from forcing water into the piping, a deluge valve is used in the water supply connection, which is a mechanically latched valve. It is a non-resetting valve, and stays open once tripped. Because the heat sensing elements present in the automatic sprinkler heads have been removed, the deluge valve must be opened as signaled by a fire alarm system. The type of fire alarm initiating device is selected mainly based on the hazard (e.g., smoke detectors, heat detectors, or optical flame detectors). The initiation device signals the fire alarm panel, which in turn signals the deluge valve to open. Activation can also be manual, depending on the system goals. Manual activation is usually via an electric or pneumatic fire alarm pull station, which signals the fire alarm panel, which in turn signals the deluge valve to open.
Pre-action sprinkler systems are known in the art for use in locations where accidental activation of the sprinkler system is undesired, such as in museums with rare art works, manuscripts, or books; and data centers, for protection of computer equipment from accidental water discharge. Pre-action systems are hybrids of wet, dry, and deluge systems, depending on the exact system goal. There are two main sub-types of pre-action systems: single interlock, and double interlock.
The operation of single interlock systems is similar to dry systems except that these systems require that a preceding fire detection event, typically the activation of a heat or smoke detector, takes place prior to the action of water introduction into the system's piping by opening the pre-action valve, which is a mechanically latched valve (i.e., similar to a deluge valve). In this way, the system is essentially converted from a dry system into a wet system. The intent is to reduce the undesirable time delay of water delivery to sprinklers that is inherent in dry systems. Prior to fire detection, if the sprinkler operates, or the piping system develops a leak, loss of air pressure in the piping will activate a trouble alarm. In this case, the pre-action valve will not open due to loss of supervisory pressure, and water will not enter the piping.
The operation of double interlock systems is similar to deluge systems except that automatic sprinkler heads are used. These systems require that both a preceding fire detection event, typically the activation of a heat or smoke detector, and an automatic sprinkler head operation take place prior to the action of water introduction into the system's piping. Activation of either the fire detectors alone, or sprinklers alone, without the concurrent operation of the other, will not allow water to enter the piping. Because water does not enter the piping until a sprinkler head operates, double interlock systems are considered as dry systems in terms of water delivery times, and similarly require a larger design area.
A sprinkler head is the component of a fire sprinkler system that discharges water when the effects of a fire have been detected, such as when a predetermined temperature has been exceeded. Each sprinkler head is held closed by a heat-sensitive glass bulb or a two-part metal link held together with fusible alloy such as Wood's metal and other alloys with similar compositions. The glass bulb or link applies pressure to a pipe cap which acts as a plug to prevent water from flowing until the ambient temperature around the sprinkler head reaches an activation temperature. Because each sprinkler head activates independently when the activation temperature is reached, the number of sprinkler heads that operate is limited to only those near the fire, thereby maximizing the available water pressure over the point of fire origin. In glass bulb-type sprinkler heads, the bulb breaks as a result of the thermal expansion of the liquid inside the bulb. The time it takes before a bulb breaks is dependent on the temperature. Below the design temperature, it does not break, and above the design temperature it breaks, taking less time to break as temperature increases above the activation temperature.
In many cases, it is desirable to generally protect a building from fire using a simple wet pipe sprinkler system, while protecting certain special rooms (e.g. water sensitive or unheated rooms) in the building using a dry pipe system. It is currently possible to install separate systems, a wet pipe system for most of the building and a separate dry pipe system for the special rooms. However, this approach increases expense, complicates maintenance and results in separated fire protection zones that must be separately controlled and monitored thereby duplicating fire protection efforts for just a few rooms.
There remains a need in the art for a simple, effective way for providing dry pipe fire protection for a smaller area (e.g. for one room) that is integratable into any other building-wide sprinkler system, including wet pipe systems.